python class reusing variable from past test cases

Question

I am having an issue in using the stack reverse method. The first test case works but the data is not deleted and it messes up the next text case.

EDIT: My IDE is Google Colab.

**OUTPUT:**

Stack Before: -2 0 1 4 5 
Stack After: 5 4 1 0 -2 

Stack Before: 1 2 3 4 5 6 
Stack After: 5 4 1 0 -2 6 5 4 3 2 1 

Stack Before: a b c 
Stack After: 5 4 1 0 -2 6 5 4 3 2 1 c b a 

Stack Before: 8 -2 55 12 -47 10 
Stack After: 5 4 1 0 -2 6 5 4 3 2 1 c b a 10 -47 12 55 -2 8

class Stack:
  def __init__(self, arr=[]):
    self.stack = arr

  def __len__(self):
    return len(self.stack)

  def __str__(self):
    res = ''
    for each in self.stack:
      res += str(each) + ' '
    return res

  def push(self, number):
    self.stack.append(number)

  def pop(self):
    self.stack.pop()
    return 

  def peek(self):
    ele = self.stack[-1]
    return ele

  def reverse(self, res = None) :
    if not res:
      res = Stack()
    if not self:
      return 
    x = self.peek()
    self.pop()
    res.push(x)
    self.reverse(res)
    return res


if __name__ == '__main__':
  # Test Case 01
  # Stack ---> -2 0 1 4 5
  # Expected Answer ---> -2 0 4 5 
  test1 = Stack([-2,0,1,4,5])
  print(f'Stack Before: {test1}')
  test1 = test1.reverse()
  print(f'Stack After: {test1}\n')

  # Test Case 02
  # Stack ---> 1 2 3 4 5 6
  # Expected Result ---> 1 2 3 5 6
  test2 = Stack([1,2,3,4,5,6])
  print(f'Stack Before: {test2}')
  test2 = test2.reverse()
  print(f'Stack After: {test2}\n')

  # Test Case 03
  # Stack ---> a b c
  # Expected Result ---> a c
  test3 = Stack(['a', 'b', 'c'])
  print(f'Stack Before: {test3}')
  test3 = test3.reverse()
  print(f'Stack After: {test3}\n')

  # Test Case 04
  # Stack ---> 8 -2 55 12 -47 10
  # Expected Result ---> 8 -2 55 -47 10
  test4 = Stack([8, -2, 55, 12, -47, 10])
  print(f'Stack Before: {test4}')
  test4 = test4.reverse()
  print(f'Stack After: {test4}')

You have issues with half of your methods, especially `__init__` with mutable default: see [this Q](https://stackoverflow.com/questions/1132941/least-astonishment-and-the-mutable-default-argument). You're missing arg in `self.reverse(res)` in `reverse` definition, so new stack is created. If you were trying to use mutable args for reason (which is valid, but not beginner-friendly), you did it wrong, and perhaps it's impossible for reusable class' `__init__`. Please, don't indent 2 spaces, 4 are PEP8-compliant and more readable. And have a look at `collections.deque` instead of list for `stack` — STerliakov, Aug 06 '22 at 21:50
And you're missing `__bool__`, so `if not self:` is always `False` (`bool(object)` is always `True` unless `__bool__` is overridden). — STerliakov, Aug 06 '22 at 21:52

score 0 · Accepted Answer · answered Aug 06 '22 at 23:39

Thanks for your help @SUTerliakov. I had to reverse engineer the geeksforgeeks solution though because my approach was wrong. I had to break the question in two seperate recursion problems to work.

    def insertAtBottom(self, item) :
        if self.isEmpty():
            self.push(item)
            return self
        temp = self.peek()
        self.pop()
        self.insertAtBottom(item)
        self.push(temp)
        return self

    def reverse(self) :
        if self.isEmpty():
            return self
        temp = self.peek()
        self.pop()
        self.reverse()
        self.insertAtBottom(temp)
        return self

python class reusing variable from past test cases

1 Answers1